General
For more than 100 years serious efforts have been made to gasify coal in situ from laboratory tests to full scale projects. Most projects failed as commercial ventures due to intense competition from petroleum and natural gas. Others failed due to technical deficiencies in processes.
The inability of the petroleum industry to keep pace with the demand for energy has focused world wide attention to future projects for in situ coal gasification. In the coal industry generally, it is expected that more emphasis will be placed on underground gasification of coal due, in part, to environmental requirements that severely restrict or prevent strip mining operations.
The ideal project for in situ gasification of coal would eliminate the perils of man power underground and would provide a clean, high heat content gas suitable for commercial and industrial uses. There are sufficient known coal deposits in the United States to provide total energy requirements for hundreds of years. Unfortunately, a substantial amount of these deposits are located at depths considered uneconomical for conventional underground mining. Improved new methods of in situ coal gasification can unlock the energy of deeply buried coal deposits.
Combustion of Hydrocarbons
The primary use of hydrocarbons fuels is in some method of combustion to release energy in the form of heat, whether it be in an automobile engine, or furnace to generate steam or hot air or the like. In the combustion process, oxygen is supplied to the fuel and the temperature is raised above ignition temperature resulting in rapid oxidation or burning, and a consequent rapid release of heat. Ignition temperatures in air are on the order of 1250.degree.F for natural gas (methane); 1200.degree.F for carbon monoxide; 1080.degree.F for hydrogen; 925.degree.F for anthracite coal; 820.degree.F for bituminous coal; and 600.degree.F for crude petroleum. The most common source of oxygen is air which contains 21% oxygen by volume, with the remainder substantially all nitrogen. In the ideal complete combustion process using air, hydrogen from the hydrocarbon fuel unites with oxygen to form water vapor, and carbon unites with oxygen to form carbon dioxide. Nitrogen from the injected air mainly robs heat from the process as it exits at exhaust or stack gas temperature. Thus the exit gases have given up their ability to oxidize further and are useless as a fuel. In actual practice, available oxygen is virtually impossible to use fully, therefore, exit gases also contain quantities of carbon monoxide, hydrogen, oxygen, methane, illuminants and the like.
Gasification of Coal
Coal is a solid hydrocarbon that contains extraneous matter such as moisture (water) and impurities (ash). For in situ gasification of coal, it is desirable to remove exit gases with the highest heat content possible, using moisture content to enhance the exit gases, and to leave the ash in place.
In burning coal above ground for its heat content, combustion is accomplished in two modes:
1. heat of the fire drives off the volatile content which burns as a gas above the fuel bed, and
2. the residual fixed carbon burns as coke upon the hearth or grate.
For in situ gasification, it is desirable to remove the lighter fractions of the volatile content as unburned gases mixed in the exit gases and to inject suitable gasifyers to decompose the burning fixed carbon into carbon monoxide, hydrogen and methane. For in situ gasification it is undesirable to permit the carbon monoxide to burn further into carbon dioxide, or for hydrogen to burn into water vapor, and for methane to burn into carbon dioxide and water. Therefore, carefully controlled incomplete combustion is essential to efficient in situ gasification of coal.
Coal Gasification Products
The content of exit gases from in situ gasification of coal is very important to the commercial success of a project, because the object is to recover exit gases with the highest heat content without the necessity of separating out the gases with no useful calorific content. Methane (CH.sub.4) is the most desirable of the exit gases because of its clean burning characteristics and high heat content (about 1,000 BTU per standard cubic foot). Carbon monoxide (CO) with a heat content of 315 BTU per standard cubic foot is desirable as is free hydrogen (H) with a heat content of 320 BTU per standard cubic foot. Undesirable exit gases include free nitrogen (N.sub.2) and carbon dioxide (CO.sub.2) because they are incapable of oxidation and, therefore, have no useful heat or calorific content as pipeline gases. The inefficiencies of using atmospheric air as the gasifier of coal are readily apparent in this typical analysis of exit gases:
______________________________________ Component Volume % ______________________________________ Carbon Dioxide 10.6 illuminants 0.2 hydrogen 8.7 oxygen 0.6 carbon monoxide 10.4 methane 2.0 nitrogen 67.5 ______________________________________
This results in a composite exit gas with a heat content of only about 90 BTU per standard cubic foot due to the high percentages of useless gases, nitrogen and carbon dioxide, with the highest percentage being nitrogen from injected air.
Methanization of Coal
Since coal is hydrogen deficient compared to petroleum and natural gas, additional hydrogen is required to increase the methane content of exit gases. By increasing working pressures in the reaction zone, increasing percentages of carbon in the form of methane occur. In experiments in Great Britain over 20 years ago the proportion of carbon appearing as methane was 14.4% at 10 atmospheres and 22.2% at 40 atmospheres. Since it is advantageous to increase the methane content of exit gases for in situ gasification of coal, elevated pressures are required in the reaction zone. Overburden above the coal bed, a disadvantage in conventional coal mining, is an advantage for in situ gasification because it provides a seal to avoid hot gas blowouts to the surface.
Earlier Gasification Projects
The first large scale attempt to gasify coal began as a government subsidized program in Russia in 1931. Much of the Russian field work involved substantial underground workings, preparing chambers underground, digging inlet and and outlet shafts, fire drifts, and the like. The advent of World War II with massive disruptions of normal trade channels coupled with partial successes with the Russian project generated interest in other countries and initiated new projects that continued into the post war years. Major projects were conducted in Great Britain, Russia, Poland, Italy, Belgium, Czechoslovakia, France, Morocco and the United States. All projects were characterized by a low calorific value of the produced gas (on the order of 100 BTU per standard cubic foot). This common characteristic stemmed from lack of control of inlet air underground resulting in inlet air bypassing the reaction or burning zone and proceeding to the exit area where unplanned burning of methane, hydrogen, and carbon monoxide occurred before withdrawal. Other problems with these projects included gas leakage, water encroachment, unplanned subsidence of the coal formation, and wide variations and fluctuations in the calorific content of the produced gas.